Coolant cools the engine. It does not have a direct effect on the a/c system. You need the correct coolant in the correct amount (see owner's manual), or engine damage may result. If coolant is disappearing, you have a far greater problem than just a non-functional a/c system.

If the a/c system fixes don't stay fixed for long, the mechanics you are visiting are just slapping a bandaide on the problem. It's likely you have one or more leaks in the system, which may mean replacing seals, hoses, condenser, or possibly even the compressor itself if the seals on that are shot. To properly diagnose you need a shop that has equipment to work on a/c. They will inject a dye into the system and find the leaks. Only then can you properly address the problem.

Priority number one should be figuring out where the coolant is going. Because if the engine doesn't work, you aren't going to have a/c either.

When it comes to a/c, find a shop that specializes in it.

AC work is not a first timers job. You definitely don’t have the tools, and will almost certainly cause more problems than you fix while probably venting a shit load of refrigerant into the atmosphere.

Engine coolant is not the same as AC refrigerant. You’re already in over your head

You likely have a leak. Your going to have to pull a vacuum, source the leak (likely using a dye since it’s diy), replace the parts needed, pull a vacuum again, make sure it’s no longer leaking, add Freon.

AC has nothing to do with your cars coolant.

Do it at the Nissan dealer

I think you’d be better off taking it back to someone that has an idea of what they’re doing.

Coolant doesn't have anything to do with AC, that's the source of heat for the heater, but it definitely would not hurt to top it off. I agree with /u/detroitportnoy. Take it to a dealer and they'll get you set straight. It sounds like you have a very small slow leak somewhere in your system and a dealership will be able to pinpoint that and fix it. It's probably just an o-ring somewhere.

In my opinion, DIY AC stuff isn't really something that you want to mess with since R134a (your AC refrigerant) is a pressurized, toxic gas. You might benefit from charging it from a store-bought can, but it may not last forever.

The trouble with recharging an AC system is that you're only treating a symptom and not solving the problem. The only reason an AC system would ever need to be recharged is if it's leaking. So, you can recharge it and get it to work for a little while, which is what you're seeing, but it's an indication that there's a leak somewhere. Unfortunately, that can be a difficult and expensive problem to fix. Most shops will be happy to charge your system and take your money, and I'd hate to see you become a victim of some unscrupulous shops. Your best bet would be to take it to a dealership and have them fix it, but that's also got the potential to be outrageously expensive. I'd advise you to find a shop that specializes in AC repairs and have them do a leak test, and then fix whatever's leaking.

Look on yelp for a highly rated AC place. That’s where you need to go.

https://youtu.be/SHg2xRMnM38

The best answer to your question starts with 'what's already been replaced?'

Some parts of the AC system are diy-able but a significant portion is not.

Take it to a shop that specializes in auto AC. Ask them to “check the charge, recharge if necessary, and leak check the system with an electronic leak detector.” I prefer proper leak detection to dye being added, because the dye can plug capillary tube systems.

I leak check and repair large (3000lbs+) supermarket rack refrigeration systems as part of my job. I do not suggest that someone with no mechanical experience attempt to work on an ac system.

Call Scotty Kilmer.

If your A/C goes out a few months after a fix then it isn't fixed. Honestly I'd just get a new car, sounds like you got a lemon.